Power modules are well-known electric circuit devices in the art. The power modules typically comprise an encapsulated package including a power semiconductor component, such as power MOSFETs and insulated gate bipolar transistors (IGBT), wired on a ceramic substrate. The electric circuit devices have been used as, for instance, an electric module for controlling electric power, e.g., an electric module for controlling a vehicle traction motor. Since the power semiconductor components manage high power, they are also known as a heat generator. So there has been conventional, various heat dissipation structures, also referred to as cooling structures or packaging structures, for power modules.
Some simple power modules have a heat dissipation structure to dissipate heat from the rear surface of a power device. Patent Document 1 discloses, for example, a conventional heat dissipation structure for power modules, comprising a heatsink located on the rear surface of a power module and a cooler such as an aluminum cooler fixed to the heatsink by a screw via thermal grease as a thermal conductive member such as silicone grease.
To follow downsizing and increasing power consumption of power modules, Patent Document 2 discloses a conventional heat dissipation structure comprising heatsinks exposed on both surfaces of a heat generator to increase the heat radiation area. The conventional structure further comprises an electric-insulating and thermal-conductive member (i.e., dielectric plate) and a cooler on the heatsinks via thermal grease or thermal conductive gel. The dielectric plate and the cooler are compressed onto the heatsink with an appropriate pressure to make them thermally contact each other. The thermal-conductive member itself located between the heatsink and the cooler is required to have a high thermal conductivity, and the thermal-conductive member electrically insulates the heatsink from the cooler. So, if the thermal grease which works to adhere to the heatsink and the cooler is not dielectric, the thermal-conductive member needs additional insulating material. Patent Document 2 discloses that the conventional heat dissipation structure would have the thermal resistance of 0.24 K/W or less to increase heat radiation performance.
Patent Document 3 discloses a conventional heat dissipation structure for power modules, comprising a layer containing whiskers, that may be carbon nanotubes or carbon filaments, aligned along the axis perpendicular to the heat radiation surface, i.e., the thermal conducting direction, to improve conventional heat dissipation members. However, the structure of Patent Document 3 has introduced a certain drawback that the whisker layer is fragile and the structure is difficult to mass-produce.
The conventional thermal conductive material often utilizes ceramic from the viewpoint of both dielectricity and thermal conductivity. However, since the ceramic material is rigid and has the solid surface, it is difficult to contact with an adherend. Even if the ceramic material is located, as a thermal conductive member, between a heatsink and a cooler and they are compressed together to adhere each other, there are air gaps at their interfaces, and the gaps spoil the heat conductivity. Therefore, the conventional art requires a layer of thermal grease to fill the gaps at the interfaces. Specifically, the conventional heat dissipation structure needs layers of thermal grease on both the interface between the heatsink and the thermal conductive member and the interface between the thermal conductive member and the cooler. Even if the provided thermal grease layers partially improved the thermal conductivity of the conventional heat dissipation structure, the grease layers themselves would typically have a thermal conductivity lower than that of the thermal conductive member, and the conventional structure requires two grease layers. Thus, even with using grease layers of the thinnest thickness, the whole conventional structure could stand further improvement on its thermal conductivity. Additionally, the conventional structure must require a process of proving thermal grease layers.